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pnmhistmap - draw a histogram for a PGM or PPM file
pnmhistmap
[-black] [-white] [-max N] [-verbose] [pnmfile]
Reads a portable
anymap as input, although bitmap (PBM) input produces an error message
and no image. Produces an image showing a histogram of the color (or gray)
values in the input. A graymap (PGM) input produces a bitmap output. A pixmap
(PPM) input produces pixmap output with three overlaid histograms: a red
one for the red input, a green one for the green input, and a blue one
for the blue input. The output is fixed in size: 256 pixels wide by 200
pixels high.
- -black
- Ignores the count of black pixels when scaling
the histogram.
- -white
- Ignores the count of white pixels when scaling the
histogram.
The -black and -white options, which can be used seperately or
together, are useful for images with a large percentage of pixels whose
value is zero or 255, which can cause the remaining histogram data to become
unreadbaly small. Note that, for pixmap inputs, these options apply to all
colors; if, for example, the input has a large number of bright-red areas,
you will probably want to use the -white option.
- -max N
- Force the scaling
of the histogram to use N as the largest-count value. This is useful for
inputs with a large percentage of single-color pixels which are not black
or white.
- -verbose
- Report the progress of making the histogram, including
the largest-count value used to scale the output.
All flags can be abbreviated
to their shortest unique prefix.
Assumes maxval is always 255. Images
with a smaller maxval will only use the lower-value side of the histogram.
This can be overcome either by piping the input through "pnmdepth 255"
or by cutting and scaling the lower-value side of the histogram. Neither
is a particularly elegant solution.
Should allow the output size to be specified.
pgmhist(1)
, ppmhist(1)
, pgm(5)
, ppm(5)
Wilson H. Bent. Jr. (whb@usc.edu).
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